In the past great importance has been attached to the study of the toxicity of silica, mainly in connection with silica's involvement in silicosis; but there has been little work concerned with the effect of silicon in normal metabolism. Until recently there has been no proof that silicon plays any definite role in vital processes, and the element has generally been considered nonessential. It was recently demonstrated in this laboratory that silicon is required for normal growth and development in the chick by feeding a low silicon diet in a trace element controlled environment. The observed effect of silicon deficiency on growth was marked, suggesting an important physiological role for this element. Thus this research project is concerned with establishing the physiological role of silicon: its function, site, and mode of action, with particular reference to its role in connective tissue metabolism. In vivo studies using chicks and rats and employing techniques developed in this laboratory to produce silicon deficiency in the young animal, and in vitro studies including isolation and quantitative analysis of connective tissue components, electron microprobe, and tissue culture are being carried out to attain these objectives.